Triggered-stroke actuator for a safety device incorporated into a motor vehicle to protect a pedestrian in the event of a frontal impact

ABSTRACT

A triggered-stroke actuator with a damped return stroke. According to the invention, the actuator houses a piston ( 15 ), and a gas generator ( 19 ) including a fast combustion pyrotechnic charge, and further houses a pyrotechnic charge ( 38 ) of combustion that is slower than the combustion of said gas generator ( 19 ).

The invention relates to a triggered-stroke actuator constituting the drive element of a safety system fitted to a motor vehicle and serving to lift the vehicle hood rapidly in the event of a collision with a pedestrian.

The invention relates more particularly to an improvement enabling the rod of said actuator to return in damped manner, and consequently to cause the hood to perform a damped reverse movement after the impact, and also, in the event of no impact, allowing the hood to be repositioned in its initial position under the effect of its own weight or on application of a moderate force, once a certain amount of time has elapsed after triggering.

Patent FR 2 878 212 describes a safety system for protecting a pedestrian on being struck by a motor vehicle. That system comprises a mechanism that enables the motor vehicle hood to be raised rapidly in the event of a collision. When such a collision occurs, the pedestrian's head frequently strikes the hood of the vehicle. The impact of the head against the hood causes the hood to be deformed. After deforming a certain amount, the hood comes into contact with the engine block and all of the rigid parts that surround the engine. It is at that moment that the pedestrian's head is subjected to the greatest level of deceleration, which can give rise to severe injuries. That is why the above-mentioned system is designed to raise the hood rapidly through a certain height so as to avoid the pedestrian, and in particular the pedestrian's head, striking the engine block after deforming the hood. The hood is raised at its rear end, beside the windshield. The hood remains fastened to the front of the motor vehicle

Thus, if such a safety system is actuated in time by appropriate detector means, it serves to raise the hood through a few tens of millimeters in a few tens of milliseconds (where 80 mm and 30 ms are representative values), i.e. very quickly after an imminent impact has been detected. The drive member of such a system advantageously comprises a triggered-stroke actuator of the type comprising a pyrotechnic charge that is fired electrically. The gas generated by the combustion of the charge pushes the piston of the actuator, which piston is associated with a rod that actuates the mechanism for raising the hood, or even directly actuates the hood itself.

After the hood has been raised, it is known to allow it to perform a damped return movement so as to accompany the strike against the hood and reduce its impact on the pedestrian. The return damping system is advantageously combined with the actuator that raised the hood. In this way, the assembly constituted by the hood, the raising mechanism, and the piston of the actuator retracts under the impact (after the hood has been raised and deformed) while being braked, and then blocked. The damper device is arranged in the body. It makes use of means that are mechanical.

One way of providing an anti-return effect with damping is to maintain the pressure chamber of the actuator under a pressure that is sufficiently high to limit its return movement. This length of time typically has a duration of 300 ms. It is not possible to envisage using a pyrotechnic charge of greater mass suitable for providing both the pressure on the piston for quickly deploying the actuator and also the continued pressure in the piston chamber. A pyrotechnic charge of that type would cause the actuator to be deployed too suddenly and would run the risk of damaging the deployment mechanism and the hood.

The person skilled in the art is therefore still looking for a device for an actuator, which device is simple, i.e. does not require a special mechanism, and enables the actuator to deploy at low force in a short period (typically 30 ms), and then that serves to maintain an anti-return force with damping over a long duration (typically 300 ms), followed by releasing the force so as to enable the actuator to return under low force, e.g. merely under the force of gravity after a long period, typically after several seconds.

The invention serves firstly to provide this damped return function of the hood by using means that are simpler.

To this end, the invention provides a triggered-stroke actuator comprising a body housing a piston linked to a rod that projects from one end of said body, and a controlled gas generator mounted in said body facing said piston and at a predetermined distance from an initial position thereof prior to triggering so as to define an expansion chamber between said gas generator and said piston, said gas generator including a fast combustion pyrotechnic charge, the actuator being characterized in that it includes a pyrotechnic charge of combustion that is slower than the combustion of said gas generator and that is housed in a location that communicates with said expansion chamber. Advantageously, the slow combustion pyrotechnic charge is fired by the combustion of said fast combustion pyrotechnic charge.

The slow combustion pyrotechnic charge may be housed in said gas generator including the fast combustion pyrotechnic charge, i.e., for example, in a plastics housing having a filamentary duct enabling said pyrotechnic charges to be initiated under control. In an advantageous second variant, said fast and slow combustion pyrotechnic charges are housed separately in the actuator, in communication with the expansion chamber. This variant serves to separate the functions of the charges and makes it possible to modify each of their functions independently, depending on the requirements of each application.

It could also happen that the safety system is actuated and consequently that the hood is raised without an accident occurring, for example as a result of an erroneous detection. It may also happen that the pedestrian does not strike the hood. Under such circumstances, it is desirable for the hood to be capable of returning to its initial position, after a certain length of time has elapsed since triggering.

The invention also makes such repositioning of the hood possible, automatically, e.g. under the effect of its own weight.

To this end, a calibrated leakage passage is formed through the piston, and possibly through the rod, in order to exhaust the gas that has driven the piston.

Advantageously, the piston and the rod are rigidly secured to each other and the leakage passage opens out through the surface of the rod into an annular space defined between the rod and the inside wall of the body.

In this way, the outlet from the leakage passage is in communication with the outside only once the actuator has been triggered. Throughout the period during which the actuator is not triggered, there is no fear of corrosion occurring, and the pyrotechnic charges are protected from the outside medium. The metal elements inside the actuator may be made of metals that are inexpensive without requiring any special anti-corrosion treatment.

The additional slow combustion pyrotechnic charge is consumed in about three-tenths of a second after deployment and it maintains a pressure inside the chamber that is sufficient to provide damped return of the hood during the accident.

In contrast, in the event of triggering not being followed by an impact, a greater length of time is needed to allow the gas to escape via the leakage passage. The piston then retracts into the actuator body and enables the hood to return to its usual position under the effect of its own weight or as a result of a moderate force being applied to the hood itself.

The invention can be better understood and other advantages thereof appear more clearly in the light of the following description of a presently preferred embodiment of a triggered-stroke actuator in accordance with the principle of the invention, given solely by way of example and made with reference to the accompanying drawing, in which:

FIG. 1 is a diagrammatic longitudinal section view of a triggered-stroke actuator in accordance with the invention; and

FIG. 2 is a graph showing the force developed by the actuator as a function of time starting from triggering.

FIG. 1 shows a triggered-stroke actuator 11 having a generally cylindrical body 13 housing a piston 15 associated with a rod 17 that projects axially from one end 18 of the body. The body 13 also houses a gas generator 19, advantageously an electrically controlled filamentary duct gas microgenerator that is mounted in the body facing the piston, at a predetermined distance therefrom. The gas generator 19 is held stationary at the other end 21 of the body by a setback 22 therein and by a rear abutment 23. An annular gasket 25 is placed between the gas generator 19 and the inside wall of the body. A breakable or disconnectable connection element 27 is interposed between the generator and the piston. It thus holds the piston 15 at a predetermined distance from the gas generator to stabilize the position of the piston 15 and the rod 17 prior to actuation and to define the initial volume of an expansion chamber 30 between the gas generator and the facing face of said piston.

The gas generator 19 comprises a fast combustion pyrotechnic charge. When the charge is fired, the gas ejected into the expansion chamber 30 causes the connection element 27 to break or disconnect and propels the piston-and-rod assembly so that said rod projects rapidly and axially from the end of the body. This movement serves to trigger a mechanism for raising the hood (not shown). In simple and advantageous manner, the rod may even be hinged directly to the hood.

At the end of the body from which the rod projects, there can be seen a setback 35 that forms a longitudinal annular space 36 between the rod and the inside wall of the body. As shown, a sealing gasket 37 is carried by the rod in the vicinity of its end that projects outwards. This gasket extends between the rod 17 and the inside wall of the body in the vicinity of its corresponding end 18. Thus, throughout the period preceding firing, the annular space 36 is isolated from the outside medium. The annular space is also isolated from the expansion chamber by means of an O-ring 39 located between said piston 15 and said body 13.

According to an important characteristic of the invention, the actuator 11 also includes a pyrotechnic charge 38 of combustion that is slower than that of said gas generator. This second charge, in the form of a compacted powder pellet, is housed in a location that communicates with the expansion chamber 30 so as to be fired by the combustion of the fast combustion pyrotechnic charge.

In the example shown, the slow combustion pyrotechnic charge is placed in a cavity 40 of the piston that opens out into the expansion chamber.

The charge 38 is held in place by a cover that is pierced in its center and that is engaged in the orifice of the cavity 40.

Advantageously, the cavity opens out in the expansion chamber 30 axially facing the gas generator.

FIG. 2 shows the result of the combination of the successive combustions of the two charges. The force communicated axially to the piston by the expansion of the gas is plotted up the ordinate axis and time is plotted along the abscissa axis. It can be seen that a very high level of thrust is communicated by the fast combustion charge for a brief period (A). This thrust exceeds 1000 newtons (N) but falls off very quickly to 500 N approximately in a duration of a few tens of milliseconds, typically 30 ms. This corresponds to the movement of the piston and thus to triggering the safety system for raising the hood. This rapid fall does not provide sufficient return damping. Nevertheless, in accordance with the invention, the slow combustion pyrotechnic charge 38 is also fired, thereby enabling a certain level of force to be imparted, which level decreases much more slowly, as shown. It is estimated that the force developed is generally sufficient and that it must decrease sufficiently slowly, over as long as several tenths of a second (B), in order to obtain the looked-for damping, with the force opposed by the piston varying over a range of a few hundreds of newtons (C).

Furthermore, according to another advantageous characteristic, a calibrated leakage passage 45 is formed in the piston and (optionally) in the rod in order to exhaust the combustion gas. This calibrated leakage passage 45 is small enough to have no significant effect during the stage in which the expansion chamber 30 is pressurized as a result of the combustion of the two pyrotechnic charges. The dimension of the passage is adjusted to make it possible subsequently for the combustion gas contained in the expansion chamber 30 to leak away at a rate that is sufficient to ensure that the return force decreases over the specified range (D) and thus that the hood returns to its position with little force, possibly under the effect of its own weight.

In the example described, where the piston and the rod are rigidly secured to each other, the leakage passage 45 opens out through the surface of the rod into the above-described annular space 36. It communicates with the cavity 40 of the piston that houses the slow combustion pyrotechnic charge 38. In addition, a filter 47 is arranged in the piston between the cavity 40 and the calibrated leakage passage 45. This filter serves to avoid particles that result from the combustion, clogging the calibrated leakage passage.

This arrangement encourages a slow decrease of pressure in the expansion chamber and consequently a slow decrease in the force developed by the piston, over a period of time that lies outside the time corresponding to damped return of the piston. This decrease in pressure and in force continues down to a value that is sufficiently small to allow the hood to return to its initial position automatically under the favorable circumstances of the hood not being damaged, with this taking place typically after a few seconds. 

1. A triggered-stroke actuator comprising a body housing a piston linked to a rod that projects from one end of said body, and a controlled gas generator mounted in said body facing said piston and at a predetermined distance from an initial position thereof prior to triggering so as to define an expansion chamber between said gas generator and said piston, said gas generator including a fast combustion pyrotechnic charge, wherein the actuator includes a pyrotechnic charge of combustion that is slower than the combustion of said gas generator and that is housed in a location that communicates with said expansion chamber.
 2. A triggered-stroke actuator according to claim 1, said slow combustion pyrotechnic charge is placed in a cavity in the piston, the cavity opening out into said expansion chamber.
 3. A triggered-stroke actuator according to claim 2, wherein said cavity opens out into said expansion chamber axially facing said gas generator.
 4. A triggered-stroke actuator according to claim 1, wherein a calibrated leakage passage communicating with said expansion chamber is formed in the piston and optionally in said rod.
 5. A triggered-stroke actuator according to claim 4, wherein said piston and said rod are rigidly secured to each other, and said leakage passage opens out through the surface of said rod into an annular space defined between the rod and the inside wall of said body.
 6. A triggered-stroke actuator according to claim 5, wherein a sealing gasket is carried by said rod in the vicinity of its free outer end, and in that, in the non-activated position of the actuator, said gasket extends between said rod and the inside wall of said body in the vicinity of its corresponding end.
 7. A triggered-stroke actuator according to claim 4, wherein said leakage passage communicates with said cavity in the piston that houses said slow combustion pyrotechnic charge.
 8. A triggered-stroke actuator according to claim 7, wherein a filter is arranged in the piston between said cavity and said calibrated leakage passage.
 9. A triggered-stroke actuator according to claim 5, wherein the annular space is isolated from the expansion chamber by means of an O-ring arranged between said piston and said body.
 10. An actuator according to claim 1, wherein is included a breakable or disconnectable connection element holding the piston at a predetermined distance from the pyrotechnic generator to act, prior to actuation, to stabilize the position of the rod and the initial volume of the expansion chamber.
 11. A triggered-stroke actuator according to claim 1, wherein said slow combustion pyrotechnic charge is placed in the gas generator including said fast combustion pyrotechnic charge, opening out into said expansion chamber. 